The Quikrete calculator says for a 24" deep hole at 12" diameter you would need 4 bags. That seems a little extreme, is that accurate?

I used 6” of gravel for drainage, 5”x5” fence post, and 1 full bag of concrete, which leaves 2-3 inches i backfilled with dirt. So not sure why they say 4 full bags per hole..

the 4x4 will change the calculator. Ive used around three bags on my post but that came to the level of surface with water shed from wood post.

A 1-foot diameter hole two feet deep has a volume of 1.6 cubic feet. Each 50-pound bag of QUIKRETE® Fast Setting Concrete Mix yields approximately 0.375 cubic feet. Therefore to fill the hole completely you will need 4.2 bags of concrete. The calculator rounds to the nearest bag, in your case to 4 bags.